1. Field of the Invention
The present invention relates to a radio communication system, and in particular to a method for encoding a status PDU in the radio communication system.
2. Background of the Related Art
Radio Link Control (RLC) is a protocol for a user equipment-UMTS Terrestrial Radio Access Network (UE-UTRAN) radio interface in a mobile communication system. The RLC sublayer consists of three RLC entities: a Transparent Mode (TM) entity, an Unacknowledged Mode (UM) entity, and an Acknowledged Mode (AM) entity. Among them, the AM entity exchanges status information using STATUS PDUs to guarantee reliable transmissions. According to this system, a transmitting AM entity retransmits RLC PDUs that are not received by the receiving AM entity.
FIG. 1 shows a model of an AM entity. This entity consists of a transmitting side and a receiving side, where the transmitting side of the AM entity transmits RLC PDUs and the receiving side of the AM entity receives RLC PDUs. The AM RLC acts either as a Sender or as a Receiver depending on the elementary procedure. A sender or a receiver can reside at either the UE or the UTRAN.
In operation, the transmitting side of the AM entity receives RLC SDUs from upper layers through an AM service access point (SAP) 10. RLC SDUs are segmented and/or concatenated into AM data (AMD) PDUs of a fixed length at a segmentation/concatenation unit 20. Segmentation is performed if the received RLC SDU is larger than the length of available space in the AMD PDU. The AMD PDU size is a semi-static value configured by upper layers and can only be changed through reestablishment of the AM RLC entity by upper layers. The AMD PDU may contain segmented and/or concatenated RLC SDUs. The AMD PDU may also contain padding to ensure that it is of a valid size. Length indicators are used to define boundaries between RLC SDUs within AMD PDUs. Length indicators are also used to define whether a Padding or Piggybacked STATUS PDU is included in the AMD PDU.
After segmentation and/or concatenation is performed, the AMD PDUs are placed in the Retransmission buffer 30 and at the MUX 40. AMD PDUs buffered in the Retransmission buffer 30 are deleted or retransmitted based on a status report found within a STATUS PDU or Piggybacked STATUS PDU sent by the peer AM RLC entity. This status report may contain positive or negative acknowledgements of individual AMD PDUs received by the peer AM RLC entity.
The MUX 40 multiplexes AMD PDUs from the Retransmission buffer 30 that need to be retransmitted and the newly generated AMD PDUs delivered from the Segmentation/Concatenation unit 20. They are then temporarily stored the multiplexed AMD PDUs at a transmission buffer 50.
The PDUs are delivered to a unit 60 that completes the AMD PDU header and potentially replaces padding with piggybacked status information. Piggybacked STATUS PDUs can be of variable size in order to match the amount of free space in the AMD PDU. The AMD PDU header is completed based on an input from an RLC Control Unit (80) that indicates the values to set in various fields (e.g. Polling Bit). Unit 60 also multiplexes, if required, Control PDUs received from the RLC Control Unit (RESET and RESET ACK PDUs) and from the Reception buffer (Piggybacked STATUS and STATUS PDUs), with AMD PDUs.
A ciphering unit (if configured) 70 ciphers the AMD PDUs. The AMD PDU header is not ciphered. Piggybacked STATUS PDU and Padding in AMD PDU (when present) are ciphered. Control PDUs (i.e. STATUS PDU, RESET PDU, and RESET ACK PDU) are not ciphered. The transmitting side of the AM entity submits AMD PDUs to the lower layer through one or two dedicated control channel(s) (DCCH) or dedicated traffic channel(s) (DTCH).
The receiving side of the AM entity receives AMD and Control PDUs through the configured logical channels from the lower layer. The AMD PDUs are then routed to the Deciphering Unit 100 by a demux/routing unit 90, where AMD PDUs (minus the AMD PDU header) are deciphered (if ciphering is configured and started) and delivered to the Reception buffer 110.
The AMD PDUs are replaced in the Reception buffer (110) until a complete RLC SDU has been received. The Receiver acknowledges successful reception, or requests retransmission of the missing AMD PDUs by sending one or more STATUS PDUs to the AM RLC peer entity, through its transmitting side. If a Piggybacked STATUS PDUs is formed in an AMD PDU, it is delivered to the Retransmission buffer & Management Unit 30 at the transmitting side of the AM entity after unit 120 removes the RLC header and extracts Piggybacked information, in order to purge the buffer of positively acknowledged AMD PDUs and to indicate which AMD PDUs need to be retransmitted. Once a complete RLC SDU has been received, the associated AMD PDUs are reassembled by the Reassembly Unit 130 and delivered to upper layers through the AM SAP 10.
In addition, RESET and RESET ACK PDUs are delivered to the RLC Control Unit 80 for processing. If a response to the peer AM RLC entity is need, an appropriate Control PDU is delivered by the RLC Control Unit 80 to the transmitting side of the AM RLC entity. The received STATUS PDUs are delivered to the Retransmission buffer and Management Unit 30 at the transmitting side of the AM entity, in order to purge the buffer of positively acknowledged AMD PDUs and to indicate which AMD PDUs need to be retransmitted.
The AM entity can use AM data (AMD) PDU and control PDUs. Among the control PDUs, STATUS PDU and Piggybacked STATUS PDUs are used by the Receiver to inform the Sender about missing and received AMD PDUs in the Receiver, and a size of the allowed transmission window. The STATUS PDU and Piggybacked STATUS PDUs are also used by the Sender to request the receiver to move the reception window and to send an acknowledgment to the Sender about reception of the request to move the reception window.
FIG. 2 shows a STATUS PDU format which is used to exchange status information between two AM entities. The STATUS PDU may include super-fields of different types. The size of a STATUS PDU is variable and upper bounded by the maximum RLC PDU size used by the logical channel through which the control PDUs are sent. Padding shall be included to conform to one of the PDU sizes used by the logical channel on which the control PDUs are sent. The length of the STATUS PDU is a multiple of 8 bits. The D/C field of 1 bit length indicates the type of an AM PDU (see FIG. 3) and the PDU type field of 3 bits indicates the control PDU type (see FIG. 4).
FIG. 5 shows a structure of a super-field (SUFI) which includes three sub-fields: type, length, and value. The length field provides variable length information, the type field has a length of 4 bits, and the value field indicates various information about the STATUS PDU (see FIG. 6).
As shown in FIG. 7, the List super-field consists of a type-identifier field (LIST), a list length field (LENGTH), and a list of LENGTH number of pairs. The four-bit LENGTH field is the number of (SNi, Li)-pairs in the super-field of type LIST. The value “0000” is invalid and the STATUS PDU is discarded. The 12-bit SNi is “Sequence Number” of AMD PDU, which was not correctly received and 4-bit Li is number of consecutive AMD PDUs not correctly received following AMD PDU with “Sequence Number” SNi.
As shown in FIG. 8, the Relative List super-field consists of a type identifier field (RLIST), a list length field (LENGTH), the first sequence number (FSN), and a list of LENGTH number of codewords (CW). The 4-bit LENGTH field is the number of codewords (CW) in the super-field of type RLIST. The 12-bit FSN field is the “Sequence Number” for the first erroneous AMD PDU in the RLIST, i.e. LENGTH=“0000” means that only FSN is present in the SUFI. The CW consists of 4 bits, where the three first bits are part of a number and the last bit is a status indicator having a value interpreted as in FIG. 9.
By default, the number given by the CWs represents a distance between the previous indicated erroneous AMD PDU up to and including the next erroneous AMD PDU. One special value of CW “0001” is defined as the “Error burst indicator,” which means that the next CWs will represent the number of subsequent erroneous AMD PDUs (not counting the already indicated error position). After the number of errors in a burst is terminated with XXX1, the next codeword will again by default be the least significant bits (LSB) of the distance to the next error.
The 3GPP standard TS 25.322 normatively defines errors and the handling of errors. A STATUS PDU or Piggybacked Status PDU including Erroneous Sequence Number is a STATUS PDU or Piggybacked Status PDU that contains a LIST, BITMAP, or RLIST SUFI. Which SUFI fields to use is implementation dependent. Bitmap SUFI is used to indicate both received and/or missing AMD PDUs. List SUFI and/or Relative List SUFI are used to indicate missing AMD PDUs only.
Encoding methods of the List and Relative List SUFIs for indicating missing AMD PDUs have respective advantages and disadvantages.
The Relative List SUFI encoding is superior to the List SUFI encoding in encoding efficiency (number of bytes/information), but it is complex because of the large number of calculations required and is not intuitive. On the other hand, the List SUFI encoding is intuitive and easy to implement because it requires relatively fewer calculations, but its encoding efficiency is less than that of the Relative List SUFI encoding.
In spite of these differences, the 3GPP standard does not specify which SUFI is used for reporting specific erroneous status, i.e., occurrence pattern of the missing AMD PDUs. Accordingly, the receiver randomly selects one of the SUFI types and encodes the SUFI with the selected encoding method for the STATUS PDU to be sent to the sender, and the sender decodes the SUFI of the received STATUS PDUs based on type sub-field of the SUFI.
In this conventional status report method, the SUFI type for indicating the missing AMD PDUs is randomly selected among the three super-field types, such that a reasonable method for selecting error occurrence pattern-adaptive SUFI type is required.